Mileage versus travelling Speed ?? Is there truely exists something like fuel economy Speed?
Mileage versus travelling Speed ?? Is there truely exists something like fuel economy Speed?
(OP)
I am just curious to know the theory behind vehicle speed versus mileage.
My mechanic advised that vehicle should be driven at optimum speed to get the maximum mileage.Low speed or High speed may result in over fuel consumption.
Is it true?
How low speed consume more fuel.
How high speed consume more fuel.
My mechanic advised that vehicle should be driven at optimum speed to get the maximum mileage.Low speed or High speed may result in over fuel consumption.
Is it true?
How low speed consume more fuel.
How high speed consume more fuel.





RE: Mileage versus travelling Speed ?? Is there truely exists something like fuel economy Speed?
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RE: Mileage versus travelling Speed ?? Is there truely exists something like fuel economy Speed?
RE: Mileage versus travelling Speed ?? Is there truely exists something like fuel economy Speed?
RE: Mileage versus travelling Speed ?? Is there truely exists something like fuel economy Speed?
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RE: Mileage versus travelling Speed ?? Is there truely exists something like fuel economy Speed?
An engine consumes fuel according to it's BSCF and the power generated.
BSFC is lower at higher loads, so effectively as the speeds increase, the BSFC decreases.
Obviously, there is a sweet spot, and either side will see an increase in consumption, but every vehicle is different.
RE: Mileage versus travelling Speed ?? Is there truely exists something like fuel economy Speed?
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Greg Locock
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RE: Mileage versus travelling Speed ?? Is there truely exists something like fuel economy Speed?
No data was taken at higher speeds for fear of governmental intervention.
No data was taken at lower speeds for fear of being run over by locals.
Mike Halloran
Pembroke Pines, FL, USA
RE: Mileage versus travelling Speed ?? Is there truely exists something like fuel economy Speed?
This is true for a spark ignition engine because lower loads are handled by throttling the engine (almost universally with a choke type throttle) and introducing pumping losses in the process. Therefore, for any given load it is best if the engine can meet that load at wide open throttle. This means that for cruising at 55mph with a drag load of 15HP, the best efficiency is achieved with an engine that makes 15 HP wide open while running at its torque peak.
A more powerful engine will be suffering from pumping losses at that speed. Going slower will require more throttling with even more pumping losses. Going faster will reduce the pumping losses, but will encounter more drag losses mainly from air drag.
For any given car with its own drag characteristic and with its own particular engine there will be a particular speed at which it gets the optimum balance and its best mileage.
RE: Mileage versus travelling Speed ?? Is there truely exists something like fuel economy Speed?
Cruising at 12 to 18 inches of mercury vacuum operating thruout the intake stroke would seem to seem to be worth about 7 psi of BMEP if totally unrecoverable.
And that would seem to be worth 10% (+/- a lot) at various points around various cruising rpm on a chart like this.
http://www.techno-fandom.org/~hobbit/cars/bmep-Fin...
Then I start to think about the effective compression ratio being about half as a result of the 0.5 atmosphere pressure in the intake.
So a 10-1 compression ratio might be more like 5-1 running under part throttle/high vacuum. That looks like about a potential 20% reduction in theoretical thermal efficiency. 60% down to 47% as shown here.
http://www.fao.org/docrep/t0512e/T0512e0v.gif
The fact that an additional 10-15 degrees or so ignition advance when cruising on the level is called for seems to confirm combustion is mighty slow, whether just from rarified (not leaner) fuel mixture or even additionally from exhaust gas dilution.
Going the "other" way with a 70-80% overdrive has been worth a minimum of 10% better fuel economy a couple of times for me. When commuting over not-too-hilly terrain with 70s and 80s era Volvos.
RE: Mileage versus travelling Speed ?? Is there truely exists something like fuel economy Speed?
"Schiefgehen will, was schiefgehen kann" - das Murphygesetz
RE: Mileage versus travelling Speed ?? Is there truely exists something like fuel economy Speed?
Ah, but to offset this, a large engine typically runs much longer gearing, allowing the engine to turn slower giving lower friction losses and requiring more throttle to be used. Low RPM, wider throttle position usually means that they can return reasonable economy vs a much smaller engine with shorter gearing and higher RPM, which gives more fricion losses and requires more throttling due to reduced torque requirement.
RE: Mileage versus travelling Speed ?? Is there truely exists something like fuel economy Speed?
In the Honda Pilot, the engine management system supposedly shuts off cylinders to get fuel efficiency.
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RE: Mileage versus travelling Speed ?? Is there truely exists something like fuel economy Speed?
RE: Mileage versus travelling Speed ?? Is there truely exists something like fuel economy Speed?
Here is a 1.8l corolla BSFC map
RE: Mileage versus travelling Speed ?? Is there truely exists something like fuel economy Speed?
There are indeed methods to offset the inefficiencies of running a powerful engine far removed from its most efficient mode, ie WOT at torque peak. But, current engines being operated like that do not match the economy of a much smaller engine operating IN its most efficient mode.
Among methods mentioned was cylinder deactivation which virtually eliminates air pumping losses in the deactivated cylinders, leaving the active cylinders to perform closer to the efficient mode of a much smaller engine and suffering mainly only from the unproductive frictional losses from the deactivated cylinders.
Low rpms also help especially if the cams and tuned elements can be varied to match the low engine speed, effectively retuning the engine to be efficient at ultra low rpms.
Modern methods may eliminate the economy disadvantages of large, powerful engines.
RE: Mileage versus travelling Speed ?? Is there truely exists something like fuel economy Speed?
Most of the curve(s) used for arguing going 55 mph, show a hump that peaks at ~55mph, but there seems to be a raft of curves that show at even slower speeds result in even better mileage, like:
What makes the difference?
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RE: Mileage versus travelling Speed ?? Is there truely exists something like fuel economy Speed?
Among other things, the losses in the diff.
These have two main parts, a relatively large constant friction torque, and then a load dependent part. The upshot of this is that low speeds (hence low loads) see much greater %age driveline losses than high speeds.
Then in manual cars you have the mismatch between the possible gear ratios and the desired engine speed. In autos you have the same, slurred by the torque converter, which adds another set of losses, until you hit the lock up clutch.
The easiest case is things like the Prius where the gear ratio can be correctly adjusted to the road speed, so for a given demand power the engine can be run at its most efficient rpm.
With a typical bsfc map and typical gearing it is very hard to get near the optimum bsfc point on the map, and the Prius suffers from the same problem, the optimum bsfc is at a huge power output compared with road load. So you have to settle for trying to get near the best sfc for a given road load power.
Cheers
Greg Locock
New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?
RE: Mileage versus travelling Speed ?? Is there truely exists something like fuel economy Speed?
"Schiefgehen will, was schiefgehen kann" - das Murphygesetz
RE: Mileage versus travelling Speed ?? Is there truely exists something like fuel economy Speed?
The Volt on the other hand probably has a bigger margin between road load efficiency and its best, so it might make sense to run the engine a bit hard and then turn it off.
The other thing that gets in the way is that you want to leave enough headroom in the battery for any regen stops.
With current battery technology a self powered (ie not plug in) hybrid's fuel economy is affected to a surprisingly small extent as the battery capacity drops (typically due to old age), apparently, which may be another way of saying that the optimum battery size might actually be quite small, if you aren't going to plug the car in. Here's a study on this, note that increasing the battery size (CD range in their termininology) REDUCES mpg once you have a certain minimum CD range, 15 miles in this example which annoyingly lacks resolution, because all you are doing is lugging extra mass around.
https://mospace.umsystem.edu/xmlui/bitstream/handl...
Cheers
Greg Locock
New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?
RE: Mileage versus travelling Speed ?? Is there truely exists something like fuel economy Speed?
RE: Mileage versus travelling Speed ?? Is there truely exists something like fuel economy Speed?
http://techno-fandom.org/~hobbit/cars/sweet/refl.h...
"you're now thinking "gee, if I lock it in at 15+ kW and burn 1.3 gallons in an hour and travel 68 miles at best, that's only 52 miles per gallon!" -- while true on paper, real life driving presents enough opportunities to glide that it generally winds up a good deal higher than that."
RE: Mileage versus travelling Speed ?? Is there truely exists something like fuel economy Speed?
RE: Mileage versus travelling Speed ?? Is there truely exists something like fuel economy Speed?
Cheers
Greg Locock
New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?
RE: Mileage versus travelling Speed ?? Is there truely exists something like fuel economy Speed?
S= six cylinder
V = V8 engine (which V8, and which equipment unknown)
http://i132.photobucket.com/albums/q8/LHSJR/59SGas...
Unfortunately Stanwood W. Sparrow died in 1952. If he had been behind the above testing, with his NACA and SAE credentials it would have borne even more weight.
RE: Mileage versus travelling Speed ?? Is there truely exists something like fuel economy Speed?
That sounds really intersting. Thanks for the data ..
RE: Mileage versus travelling Speed ?? Is there truely exists something like fuel economy Speed?
RE: Mileage versus travelling Speed ?? Is there truely exists something like fuel economy Speed?
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RE: Mileage versus travelling Speed ?? Is there truely exists something like fuel economy Speed?
Jay Maechtlen
http://www.laserpubs.com/techcomm
RE: Mileage versus travelling Speed ?? Is there truely exists something like fuel economy Speed?
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RE: Mileage versus travelling Speed ?? Is there truely exists something like fuel economy Speed?
Oldsmobiles of the time, with their relatively large displacement but high-compression V8 engines were noted for good fuel economy relative to their market position. When comparing automatic transmission equipped vehicles, the Hydramatic transmission was also part of the equation for good fuel economy.
"Schiefgehen will, was schiefgehen kann" - das Murphygesetz
RE: Mileage versus travelling Speed ?? Is there truely exists something like fuel economy Speed?
I think some of the Studebaker ads of around that time talked about the V8 and 6 being best in their classes, but intentionally did not compare the 6s and 8s.
Not sure of the year, but I think Studebaker converted the flathead 6 to OHV around 59/60.